Issue #2536906 by aspilicious: Warning when saving third party settings while no settings available

core/modules/field_ui/src/Form/EntityDisplayFormBase.php

@@ -567,7 +567,7 @@ protected function copyFormValuesToEntity(EntityInterface $entity, array $form,
         if ($form_state->get('plugin_settings_update') === $field_name) {
           // Only store settings actually used by the selected plugin.
           $default_settings = $this->pluginManager->getDefaultSettings($options['type']);
-          $options['settings'] = array_intersect_key($values['settings_edit_form']['settings'], $default_settings);
+          $options['settings'] = isset($values['settings_edit_form']['settings']) ? array_intersect_key($values['settings_edit_form']['settings'], $default_settings) : [];
           $options['third_party_settings'] = isset($values['settings_edit_form']['third_party_settings']) ? $values['settings_edit_form']['third_party_settings'] : [];
           $form_state->set('plugin_settings_update', NULL);
         }

core/modules/field_ui/src/Tests/ManageDisplayTest.php

@@ -163,6 +163,11 @@ function testFormatterUI() {
     $edit = array('fields[field_test][type]' => 'field_no_settings', 'refresh_rows' => 'field_test');
     $this->drupalPostAjaxForm(NULL, $edit, array('op' => t('Refresh')));
     $this->assertFieldByName('field_test_settings_edit');
+
+    // Make sure we can save the third party settings when there are no settings available
+    $this->drupalPostAjaxForm(NULL, array(), "field_test_settings_edit");
+    $this->drupalPostAjaxForm(NULL, $edit, "field_test_plugin_settings_update");
+
     // Uninstall the module providing third party settings and ensure the button
     // is no longer there.
     \Drupal::service('module_installer')->uninstall(array('field_third_party_test'));